Growing demand for bandwidth is fueling the need for advances in optically based telecommunications. Many of these advances are provided by new developments in optical components. Most of the currently available solutions have slow switching times, in the order of milliseconds, high insertion loss and lack scale-up capabilities. On the other hand, there are a number of technologies that have high switching speed, but are fundamentally limited in the number of ports. Following is a short review of existing optical component technologies for both active (e.g. switching) and passive (e.g. splitting) applications.
MEMS (Micro Electro Mechanical Systems) technology is a relatively mature and cost effective technology, in use for more than 10 years for commercial applications. There are two primary types of MEMS optical components or devices: 2-D (2 dimensional architecture) and 3-D (3 dimensional architecture) devices. 2-D or digital MEMS devices operate in an “on/off” fashion. The typical architecture for the 2D MEMS device is the digital crossbar. 3-D MEMS devices involve a much more intricate design and driver set. 2-D MEMS components suffer from high insertion loss and cross talk, and cannot be easily scaled to a high port count as a result of the fact that the number of nodes of the crossbar equals the square of the port count number. 3-D MEMS components suffer from complex control system and high production cost, and therefore have low cost effectiveness. Both technologies operate in the millisecond switching speed domain, and are therefore not suited for IP routing and other high speed switching applications. A free-space mode of light propagation is common to all MEMS based switching solutions.
Another technology for optical switching is the “Photonic Switching Platform”. This technology was derived from the ink injection technology, and uses oil as an injection fluid in a two-dimensional switching device in which an oil bubble does the switching as a function of temperature changes. Operational instabilities, high insertion loss and operational speed of milliseconds are the main drawbacks of this technology.
A number of vendors are developing optical switches based on liquid crystal technology—the same basic technology behind laptop computer and other electronic displays. A typical liquid crystal switch works by using an electrical current to alter the polarization modes of light passing through the fabric. Limitations of liquid crystal technologies include inability to scale to high port counts in practical applications, and high polarization dependence loss and insertion loss.
Optical switch fabrics with fast switching speeds but with small port counts are under development by the industry, mainly using electro-holography or thermo-optical approaches for the switching mechanism. At present, no commercial, fast switching (sub-microsecond) switches with high port count are available, due to the inability to scale-up, and the relatively high production costs of these two technologies.
All existing optical telecommunication systems are based on optical fibers as the transmission medium. Integrated (as well as hybrid) optical components use optical waveguides in both active and passive applications. Optical waveguides can be classified into two groups or types: solid waveguides and hollow waveguides (hereinafter “HW”s). Existing waveguide-based optical switching systems include exclusively solid waveguides (“first group”) that operate on the principle of a differential refraction index between the waveguiding path and the surroundings: the waveguiding path (in short, the “waveguide”) has a higher index of refraction than the surrounding environment. In these devices the switching occurs between two waveguides. The architecture of devices based on this technology requires many stages of “cascading” for the multiple ports devices resulting in the cross talk. In addition, geometrical limitations on the waveguides in the coupling region lead to large footprint areas or alternatively to small port counts for the available fabric dimension. This type of waveguide, while having the advantage of small losses at bends, additionally has a number of disadvantages: it cannot carry high-energy signals, and it experiences losses at its connections with the external world, e.g. to external fibers.
The second type of waveguides is hollow waveguides with refractive coatings. A “hollow optical pipeline made of reflective pipes” was first proposed by Charles C. Eaglesfield in January 1961. HWs are described extensively in the literature, for example in “Optical fiber communications: devices, circuits and systems” by M. J. Howes and D. V. Morgan, John Wiley&Sons, E. A. J. Marcatili and R. A. Schmeltzer, “Hollow metallic and dielectric waveguides for long distance optical transmission and lasers”, Bell Syst. Tech. J, V43, 1964, pp. 1759-1782, and more recently in T. Miura, F. Koyama, Y. Aoki, A. Matsutani and K. Iga “Hollow Optical Waveguide for Temperature-Insensitive Photonic Integrated Circuits” Jpn. J. Appl. Phys. Vol. 40 (2001), L688-L690, Part 2, No. 7A.
Hollow waveguides are not used for communication purposes, mainly because they have high losses as a result of the waveguide bending. Nevertheless, hollow waveguides have some advantages when compared with conventional waveguides, namely, ability to carry high energy signals, and absence of losses in the connections between fiber and the waveguide. Hollow waveguides are mainly used in medical applications in the 10-20 μm IR wavelength range, due to their ability to transmit high energy densities.
In view of the disadvantages of existing technological solutions for optical components based on solid waveguides, it would be highly advantageous to have an optical switching system based on hollow waveguides that can carry high energy signals and reduce the losses at its interface with external fibers and similar elements. Another advantage of hollow waveguide based optical switching systems is the ability to integrate moving or non-moving switching elements inside the waveguide.